This invention relates to an automatic transmission of an automobile, particularly to a controlling hydraulic circuit preferably applicable to a control apparatus of a multistage automatic transmission comprising a combination of a main shifting unit and a sub-shifting unit, and more particularly to an accumulator back pressure control apparatus disposed in the hydraulic circuit.
Generally, an automatic transmission includes a torque converter and a planetary shift gear mechanism. The shift gear mechanism comprises an overdrive (O/D) or underdrive (U/D) planetary gear unit, a front planetary gear unit and a rear planetary gear unit. The shift gear mechanism obtains four forward stages and one rear stage by employing two solenoid valves and three shift valves.
Heretofore, the aforementioned automatic transmission is provided with accumulators for an O/D direct clutch C.sub.o, O/D brake B.sub.o, direct clutch C.sub.2 and second brake B.sub.2 in order to reduce shock, i.e, shift shock at the time when the respective clutches and brakes are engaged. Further, the respective accumulators for B.sub.o, C.sub.2 and B.sub.2 are subjected to oil pressure at back pressure chambers thereof from an accumulator control valve, so that a shift shock associated with a low throttle opening is more reduced.
The aforementioned accumulator back pressure is controlled by only a throttle pressure and comprises one kind of hydraulic characteristics irrespective of a gear shift position. By this, all accumulators, i.e. engaging hydraulic characteristics of a hydraulic servo for the respective friction engaging elements, are controlled. Because of the foregoing, a most proper accumulator back pressure can not be fed for each hydraulic servo for effecting an engaging operation when each of the shifting operations is effected. The foregoing is an obstacle for obtaining a smooth shifting operation with less shifting shock.
Particularly in recent time, there has been proposed an automatic transmission for obtaining a multistage shifting through various combinations of engaging operations of clutches and brakes. In such an automatic transmission, a shifting shock is a large problem for an accumulator back pressure comprising one kind of hydraulic characteristic as mentioned above.
For example, as disclosed in a Japanese Laid-open Patent Application No. 57(1982)-37140, there has been proposed an automatic shift control apparatus for obtaining six forward stages of a shifting stage, wherein three solenoid valves and three shifting valves are provided. A sub-shifting unit which can shift between overdrive and a direct-connection stage is used in combination with a main shifting unit which can shift between first, second and third speed stages. In this case, however, if the conventional accumulator back pressure control apparatus is used as it is, there arises such a problem as follows. That is, when the main shifting unit is up shifted (first speed to second speed) with the sub-shifting unit held in the overdrive state, a torque to be transmitted to the main shifting unit becomes small. Due to the foregoing, if the oil pressure is set the same as in the case when the sub-shifting unit is held in the directly connected state, the brake B.sub.2 (and clutch C.sub.2) capacity becomes excessive and a large shift shock is produced. Similarly, when the sub-shifting unit is up shifted (direct connection to O/D) with the main shifting unit held in the low speed (first speed) state (accordingly, first speed to second of a transmission as a whole), if the oil pressure is set the same as in case when the sub-shifting unit is up shifted with the main shifting unit is held in the high speed (direct connection) state (fifth speed to sixth speed of a transmission as a whole), the brake B.sub.o capacity of the sub-shifting unit becomes excessive and a large shift shock is produced.